Don't include PiP activity in app transit

When an activity in ActivityEmbedding split with another being paused,
the PiP activity's visibility may be changed to invisible and visible
again. We don't want to include such case in the app transition
animation.

Bug: 225371112
Test: verify with PiP app with ActivityEmbedding
Change-Id: I2bb781fb362518f5f11c4c8ad65eb5d2f33a5dfd